Three forms of the IL-2 receptor have been reproted: low, intermediate, and high affinity. Each form corresponds to the expression of two different receptor subunits: p55-alpha chain and p75-beta chain. Sole expression of p55 yields low affinity receptors, sole expression of p75 yields intermediate affinity receptors, and co-expression of both subunits yields high affinity receptors. Resonance energy transfer studies have suggested the presence of another peptide subunit of the IL-2 receptor with an approximate molecular mass of 95,000 Daltons. The requirement of another protein to form a functional IL-2 receptor is supported by experiments in which fibroblasts transfected with p75 cDNA express the protein on the surface but do not bind IL-2, whereas transfected T-cells express p75 and bind IL-2. We have discovered and characterized the presence of a putative new IL-2 receptor subunit with a molecular mass of 95,000-110,000 Daltons. This subunit is present in low, intermediate, and perhaps, in high affinity receptor complexes. Its presence is required to form intermediate affinity receptors with p75, but it is not necessary to obtain IL-2 binding to p55. The p95 protein, termed the gamma subunit of the IL-2 receptor, is not ICAM1, as determined by monoclonal antibody competitor studies. This project was terminated in November 1990 when Dr. O.R. Colamonici left the NIH to go to the University of Chicago.